Scion Instruments 436-GC Service Manual page 81

436-GC/456-GC 610 Hardware description
The remaining references voltages (+0.625V and +0.156V) have low precision and stability, being
generated by a resistive divider of 1% resistors (R39, R41, and R42). These voltages are used only as
transfer standards for calibrating the higher gains of the PGA. Calibration begins by reading the +10V
reference on gain 1 and proceeding to higher gains and lower reference voltages. Each reference is
measured at a lower gain, which has already been calibrated, immediately before being used to calibrate
higher gains, so high accuracy and stability are not needed.
Multiplexers and input sources.
The primary input multiplexer, U16, at the input of the PGA can handle the ±10V signals from the detectors
and the +10V reference. All of the remaining signals are scaled to 5V or less, and are selected by the
secondary multiplexers (U4, U10, and U15) which
connect to inputs of U16. The secondary multiplexers are less expensive devices which cannot handle
voltages exceeding ±5V.
While the multiplexer channel is selected by software, the actual value output to lines MMUXO-5 is double-
buffered and delayed by hardware. Like the BP/UP\ signal mentioned above, the value in the output latch
which drives the multiplexers is loaded from a buffer when DRDY\ goes low. This setting is then used for
the next conversion about lms later. Software reloads the buffer during the interrupt routine, which is
triggered by DRDY\, with the value needed for the conversion after the next one.
The RTD probes are excited from the +5V reference through 1K±0.1% resistors. Current through the RTDs
varies between 4 and 5mA over the measured temperature range. One section of the excitation resistor
network (R3) feeds a fixed resistor, R40. This is used to verify the operation of the complete temperature
measurement circuit by reading this ADC channel during run time.
The RAM backup battery voltage is read through resistor R38, which limits the discharge current when the
power is off. (Negligible current is drawn when multiplexer U10 has power applied.) C42 reduces the noise
which is picked up due to the very high source resistance of R38.
The barometric absolute pressure sensor (MP1 on schematic sheet 1) is used to compensate the
electronic flow control system for changes in barometric pressure. Its output voltage at pin 1 depends upon
the reference voltage at pin3 and the absolute pressure according to the following formula:
= V * (.009 * P - .095),
V
out ref
where P is in kilopascals, and voltages are in volts. Normal atmospheric pressure at sea level is about
103kPa, and the reference voltage is 5 volts, so the expected value of Vout under these conditions would
be about 4.6V. Atmospheric pressure usually remains within 3% of its nominal value with changes in
weather, but decreases by about 3.4kPA for each thousand feet increase in elevation. Thus, voltage
readings at pin 1 of MP1 should be between 4.4 and 4.8V at sea level, decreasing by 150mV per thousand
feet of elevation above sea level.
Proper operation of the ADC, PGA, and two of the input multiplexers is guaranteed by a successful self-
calibration. However, this procedure could not detect a failure of the +10V reference. For this reason, the
+5V digital supply voltage is read through multiplexer U 10. Reference operation is tested, but precision is
not, since the test is good only to ±5%. Reading the -5V supply through the same multiplexer guarantees
that a power supply failure could not cause erroneous readings.
Page: 80
436-GC/456-GC Service Manual Revision B February 2019
SCION Instruments